1. Field of the Invention
This invention relates to a strain measuring device for determining a magnitude of torque applied to a passive member and an amount of strain caused by the torque applied.
2. Description of the Prior Art
Generally, when an external force is applied to a passive member of magnetic material, it is deformed by the external force and its magnetic permeability is varied according to the amount of strain on the material. Therefore, the amount of strain can be determined by detecting changes in the magnetic permeability of the passive member which is subjected to magnetic fluxes.
Illustrated in FIG. 1 is a conventional strain measuring device as disclosed in Japanese Patent Laid-Open Publication No. 57-211030. In FIG. 1, reference numeral 1 denotes a cylindrical passive member which receives a torque applied by an external force, 2 denotes a pair of band-like magnetostrictive layers fixed on their passive member 1 so as to change the magnetic permeability according to the amount of internal strain resulting from the torque applied, and 3 denotes a pair of detector coils disposed around the magnetostrictive layers 2 so as to detect the changes of the magnetic permeability thereof. The respective magnetostrictive layers are constituted by a plural number of band-like or short strip-like elements disposed symmetrically at angles of +45.degree. to the axis of the passive member 1.
In operation, when an external torque is applied to the passive member 1, principal stresses each having a principal axis in the direction of the longitudinal axis of the respective magnetostrictive layers 2 occurs. These principal stresses are, for example, a tensile force for one group of elements of magnetostrictive layers 2 and a compressive force for the other group of elements of magnetostrictive layers 2. Generally, mechanical stresses cause a magnetic material with a magnetostriction constant of other than zero, to change magnetic properties, so that changes in its magnetic permeability occur as mentioned above. This phenomenon to convert mechanical energy into electric energy is utilized in a so-called magnetostrictive transducer, and is caused by the Villari effect that magnetic permeability of magnetic material changes due to mechanical deformation thereof. It is also known that, in case of a magnetostriction constant, which quantitatively expresses a rate of magnetostriction, being positive, the magnetic permeability is increased where tensile force is applied and reduced where compressive force is applied. Inverse results are obtained in case of a negative constant. Accordingly, mechanical deformation due to a torque externally applied can be detected as changes in magnetic permeability of the magnetostrictive layer 2, and the permeability changes can be detected as changes in magnetic impedance by means of the detector coils 3, so that the torque applied to the passive member 1 and accompanying strain can be measured.
In the conventional strain measuring device as described above, the magnetic fluxes generated by energizing the detector coils 3 pass through the magnetostrictive layers 2, so that the changes in permeability resulting from the strain in the magnetostrictive layer 2 can be detected as changes in output of the detector coil 3. However, since such magnetic fluxes also permeate into the passive member 1 under the magnetostrictive layers 2 which member is generally formed out of ferromagnetic steel material such as SS41, SPCC or the like, there has been a problem that errors in measurement occur due to irregularities in magnetic characteristics and singular changes caused by magnetization of the passive member 1.
Besides, there has been another problem that accurate measurement of strain is often hindered by the thermal stresses due to a large difference in linear expansion coefficient between the passive member 1 and the magnetostrictive layers 2 fixedly secured on the surface of the passive member 1 which thermal stresses are superimposed upon the stresses resulting from the strain to be measured.
There has been still another problem that the magnetostrictive layers 2 of short strip-like forms are relatively hard and not easy to shape by machining.